Serotonin (5-HT), an important neuromodulator in the brain, is found throughout the cerebellum, a region critically involved in motor coordination and motor learning. Despite numerous physiological studies describing various modulatory actions of 5-HT on cerebellar neurons, there is still no meaningful account of how 5-HT modulation alters circuit-level functioning and, in turn, modifies behavior. I propose to examine the effects of cerebellar 5-HT on the performance of two simple eye movements, the vestibulo-ocular reflex (VOR) and optokinetic reflex (OKR) in mice. The neural circuits underlying these oculomotor behaviors are known in detail and involve the floccular regions of the cerebellum. Additional experiments will examine the effects of 5-HT on circuit-level processing of neural signals related to the VOR and OKR by measuring the output cells of the cerebellar cortex, the floccular Purkinje cells. Lastly, I will investigate the role of cerebellar 5HT to the induction of different forms of VOR learning.